Various network-based services are used to provide recommendations to users and therefore may be called recommenders (e.g., recommendation services). A recommender may constitute all or part of a network-based service that is provided by a network-based system (e.g., one or more machines configured to provide some or all of the network-based service). Examples of network-based systems include commerce systems (e.g., shopping websites or auction websites), publication systems (e.g., classified advertisement websites), listing systems (e.g., wish list websites or gift registries), transaction systems (e.g., payment websites), and social network systems (e.g., LinkedIn®, Facebook®, or Twitter®). A recommendation may be provided by a recommender to a user of a recommendation service, and the recommendation may take the form of an explicit recommendation (e.g., a message or alert that recommends a product, a service, or a person) or an implicit recommendation (e.g., a search result that is sorted, ranked, or presented in a manner that highlights a recommended product, service, or person).